1. Field of the Invention
This invention relates generally to apparatus for transmitting data between distant locations and more specifically to automatic digital modems which transmit data on telephone lines.
2. Description of the Prior Art
When data, such as inventory data, is to be transmitted from a first location to a second location, it is a common practice to provide a modulator-demodulator or modem at each of the locations. Telephone lines are commonly used to provide a communication channel between the modems. The modem at the first location, acting as a transmitter, modulates the data and transmits an analog signal through the communication channel to the modem at the second location. The modem at the second location, acting as a receiver, demodulates the analog signal to reproduce the data transmitted.
The characteristics of the communication channel, such as a telephone line, typically cause significant attenuation and delay distortion in the analog signal so that when the analog signal is demodulated, the data is inaccurate. To correct for these distortions, the receiver at the second location is typically provided with an equalization network including at least one equalizer having taps with variable multiplying coefficients. By sequentially multiplying the data in the equalizer, the intersymbol interference can be reduced to correct for the delay and attenuation distortion caused by the communication channel.
In the past, the incoming data has been random or unknown so that the equalizer has had to learn the overall impulse response of the channel by correlating the incoming symbols with their expected values. This correlation has produced error signals which have been used to adjust the multiplying coefficients of the taps in the equalizers. This procedure is disclosed and claimed in applicants' copending application for an EQUALIZER, Ser. No. 405,290 filed on Oct. 11, 1973, now U.S. Pat. No. 3,906,347 and assigned of record to the assignee of record of the present application. Although this procedure works well for updating the multiplying coefficients of the equalizer, it is particularly time consuming for use in initially setting the taps. Until the taps are correctly adjusted, any data transmitted may be erroneous so that a tedious equalization procedure can significantly reduce the data throughput of the system.
This limitation is particularly apparent in multipoint communication networks including a master station and a plurality of slave stations. The receiver at the master station must adapt to the characteristics of a different telephone channel each time it communicates with a different one of the slave stations. In a system including 20 slave stations, a single polling of the network could consume as much as forty seconds in merely adjusting the equalizer taps.
In the following articles it was disclosed that the initial setting of the equalization taps could be speeded up by transmitting isolated impulses.
Chang, R. W., "A New Equalizer Structure for Fast Start-Up Digital Communication", B.S.T.J., Volume 50, Number 6, July-August 1971. PA1 Chang, R. W. and Ho, E. Y., "On Fast Start-Up Data Communications Systems Using Pseudo-Random Training Sequences," B.S.T.J., Volume 51, Number 9, November 1972.
Due to the complexity of implementation hardware, the methods disclosed in these articles are only of academic interest and not applicable in any practical modems. In order to transmit the isolated impulses, the data transmission must first be stopped and then special circuitry must be used to both modulate and demodulate the signal. Any stoppage in the transmission of data of course significantly reduces the data throughput of the system. This problem is compounded in a multipoint communication network since the transmission of isolated impulses to correct the equalization of a receiver at a single slave station would mean that the transmission or data to all of the slave stations would be stopped.